1.Field of the Disclosure
The present disclosure relates generally to electrophotographic image forming devices, and more particularly to a method of curing an overcoat layer placed on the outermost surface of an organic photoconductor drum used in an electrophotographic image forming device. The disclosed method of curing the overcoat is a two-step process. The first curing step applies either ionizing irradiation, such as with an electron beam (‘EB’) or by gamma rays or applies non-ionizing irradiation such as ultraviolet (‘UV’) light to the overcoated photoconductor drum. A mask or shield is sized to be placed over the print area of the initially cured photoconductor drum, thereby exposing the outermost edges of the photoconductor drum. The masked photoconductor drum is then exposed to a second curing step using non-ionizing irradiation such as ultraviolet (‘UV’) light. This second curing step surprisingly increases the edge-wear resistance of the photoconductor drum without altering the discharge of the photoconductor drum. Increasing the edge-wear resistance of the photoconductor drum extends the life of the photoconductor drum in direct-to-paper printing applications.
2.Description of the Related Art
Organic photoconductor drums have generally replaced inorganic photoconductor drums in electrophotographic image forming device including copiers, facsimiles and laser printers due to their superior performance and numerous advantages compared to inorganic photoconductors. These advantages include improved optical properties such as having a wide range of light absorbing wavelengths, improved electrical properties such as having high sensitivity and stable chargeability, availability of materials, good manufacturability, low cost, and low toxicity.
While the above enumerated performance and advantages exhibited by an organic photoconductor drums are significant, inorganic photoconductor drums traditionally exhibit much higher durability—thereby resulting in a photoconductor having a desirable longer life. Inorganic photoconductor drums (e.g., amorphous silicon photoconductor drums) are ceramic-based, thus are extremely hard and abrasion resistant. Conversely, the surface of an organic photoconductor drums is typically comprised of a low molecular weight charge transport material, and an inert polymeric binder and are susceptible to scratches and abrasions. Therefore, the drawback of using organic photoconductor drums typically arises from mechanical abrasion of the surface layer of the photoconductor drum due to repeated use. Abrasion of photoconductor drum surface may arise from its interaction with print media (e.g. paper), paper dust, or other components of the electrophotographic image forming device such as the cleaner blade or charge roll. Of particular interest in direct-to-paper printing applications is the abrasion of the photoconductor drum surface due to the repeated interaction with the edge of the print media, typically known as paper edge wear. The abrasion of photoconductor drum surface degrades its electrical properties, such as sensitivity and charging properties. Electrical degradation results in poor image quality, such as lower optical density, and background fouling. When a photoconductor drum is locally abraded, images often have black toner bands due to the inability to hold charge in the thinner regions. This black banding on the print media often marks the end of the life of the photoconductor drum, thereby causing the owner of the printer with no choice but to purchase another expensive photoconductor drum or a new image unit, or in some cases, the whole cartridge altogether. The useful life of an organic photoconductor drums are extremely variable. Usually, organic photoconductor drums sized 30 mm in diameter can print between about 5000 to 50,000 pages before they have to be replaced.
Increasing the life of the organic photoconductor drum will allow the photoconductor drum to become a permanent part of the electrophotographic image forming device. In other words, the organic photoconductor drum will no longer be a replaceable unit nor be viewed as a consumable item that has to be purchased multiple times by the owner of the electrophotographic printer. Photoconductor drums having an ‘ultra long life’ allow the printer to operate with a lower cost-per-page, more stable image quality, and less waste leading to a greater customer satisfaction with his or her printing experience. An organic photoconductor drum sized 30 mm in diameter having an ultra long life can print at a minimum 150,000 pages before the consumer has to purchase a replacement.
To achieve a long life photoconductor drum, especially with organic photoconductor drum, a protective overcoat layer is coated onto the outermost surface of the photoconductor drum. A protective overcoat layer formed from a silicon material has been known to improve life of the photoconductor drums used for color printers. However, this overcoat layer does not lead to the robustness needed for edge wear in organic photoconductor drums used in direct-to-paper printing. Photoconductor overcoat formulations comprising a crosslinked layer of hexa-urethane acrylate and a crosslinkable charge transport molecule are disclosed in U.S. Pat. Nos. 8,940,466, 9,360,822, 9,417,537 and 9,417,538, which are assigned to the assignee of the present application and are incorporated by reference herein in their entirety. While the use of these urethane acrylate overcoat formulations have reduced the drum wear overall in an organic photoconductor, the improvement in paper edge wear resistance in the organic photoconductor drum in direct-to-paper printing has not been realized. This disclosure aims to further improve the paper edge wear resistance of overcoated photoconductor drums by employing a second curing step in conjunction with a mask placed over the print area of the photoconductor drum. An example of the mask is made of an aluminum sheet. The protective mask is sized to be equal the print areas of the photoconductor drum, thereby exposing the outermost edges of the photoconductor drum. The mask is placed over the overcoat after the first curing step and then the exposed edges of the overcoated photoconductor drum are subject to a second UV curing step. The purpose of the mask is to enhance the degree of polymer cross-linking in the overcoat in the paper edge area while not altering the degree of polymer cross-linking in the overcoat in the print area. Importantly the electrical discharge in the print area remains unchanged as compared to the electrical discharge in the print area of the single-step cured overcoat, however the wear resistance in the paper edge is greatly enhanced when this second curing step is performed.